In a typical optical system, in simplified terms, there is an object, a lens to focus an image of the object, and an image plane onto which the image of the object is projected from the lens. The magnification of the projected image in such a system may be determined by the spacing of the object from the lens with the lens spaced from the image plane to obtain good focus. In that system, magnification can be changed simply by moving the object relative to the lens to increase or decrease the distance between them.
One application for a projection optical system is in a reduction stepper. Thus, the object is a reticle having a pattern that is to be projected onto a wafer that is located at the image plane. Thus, the magnification of the image from the reticle that is projected onto the wafer can usually be easily changed by merely changing the coordinates between the wafer and the lens, and the reticle and the lens. That is, by simply moving the reticle with respect to the lens and the wafer with respect to the lens in a fixed relationship with respect to each other, the magnification of the image projected onto the wafer is easily changed. Further, the alignment of targets at the edge of a field on the wafer are automatically checked, and if the projected targets from the reticle are not projected to the same scale as those on the wafer, there is a magnification error. Thus, in such a system, the magnification of the projected image from the reticle may be automatically changed before the wafer is exposed. Note, however, that the magnification change in the x and y directions will always remain the same and cannot be altered by different amounts in this optical configuration.
In some optical systems, however, the magnification between the image and the object, or reticle, can not be varied. One such system is a Wynne Dyson optical system that is telecentric on both the reticle side and the wafer side. In such a system, various things can be varied in the lens system, without effecting the magnification ratio. The magnification stays stuck at 1.000000 . . . to less than a part per million.
The need for the ability to make small adjustments in the magnification in all stepper systems is caused by substantially the same reasons. Namely, through the processing of a wafer, multiple layers of different materials each patterned with a different reticle pattern are laid-down on the wafer. Thus, the wafer has an initial scale when it receives the first layer, and as each layer is added to the wafer, the wafer goes into either tension or compression with the addition of each layer, so that with each step of the process the wafer changes scale by a few parts per million (ppm). Perhaps as much as 10 ppm in an extreme case. To aggravate the problem even more, it is highly probable that each change in scale of the wafer can be, and usually is, slightly different in the x direction than it is in the y direction.
As a result of the scale changes from the previous layers, the next pattern from the reticle may not fit very well over the previous patterns. Thus, there may be a size or magnification mismatch of the reticle pattern when projected onto the wafer.
In all types of stepper systems, and in optical systems in general, there are many other things that can cause a magnification change. For example, a reticle is generally made of fused-silica that expands and contracts very little with changes in temperature, however, a semiconductor wafer has a relatively high coefficient of expansion with changes in temperature. Since the wafer and reticles are not necessarily at the same temperature each time they are placed in the stepper, the expansion of the wafer can become critical, even if the temperature change is only 1.degree. C., plus or minus. For example, a typical wafer will undergo approximately a 2.4 ppm change in scale for a temperature change of 1.degree. C.
Additionally, when a lens is built, its parts are never made perfectly, or identically each time, which can result in distortion. Thus, in a stepper system the reticle pattern that reaches the wafer is not exactly the same as the original pattern--it is distorted in some way. A common form of that distortion is that the magnification may not be exactly the same in the x and y directions. For example, the magnification could be a few ppm greater in the x direction and a few ppm less in the y direction, or some other variation between the two directions.
In the telecentric Wynne Dyson optical system employed in Ultratech Stepper, Inc., stepper systems, there could be a cylindrical shape built into the internally reflecting surfaces in the fold prisms since it is impossible to make a perfectly flat surface. This will generate a magnification error which cannot be easily corrected.
Thus it can be seen that in all optical systems, whether it is a stepper projection system or some other system, there are likely to be inherent magnification errors that may not be easily compensated for in the prior art. Correction techniques and devices suitable for unscanned, full field projection systems usually make the same correction in both the x and the y direction, regardless of whether there may be more correction needed in one direction than in the other.
There are other prior art 1:1 magnification systems, for example scanners where initially there was no provision to compensate for magnification changes. The errors were simply lived with, as they have been with the telecentric Wynne Dyson type optical stepper systems. However, as the size of wafers grew bigger and the reticle patterns became finer, a built-in technique was devised for the Micralign Models 500 and 600 generation of scanners to vary the magnification to compensate for magnification error. In Micralign scanners the magnification is changed by performing two different operations. In a scanner, the illumination on a wafer is constrained to a narrow slit spanning the width of the wafer. To change the magnification in the along the slit direction, a pair of optical shells near the intermediate image plane, can be moved resulting in a variation the magnification in the along the slit direction. And then to change the magnification in the scanning direction, the cross slit direction, the reticle can be gradually moved with respect to the wafer as the wafer moves across the slit. That is, as successive parts are copied, the reticle is continuously advanced with respect to the wafer resulting in a change in magnification in the cross slit direction. This technique for scanners, however, is not applicable to a full field telecentric Wynne Dyson type optical stepper system because: a) scanning is not performed; b) there are no shells in the optical path that could be moved to vary the magnification in either direction; and c) imagery is done simultaneously over the whole field and not in a small localized region defined by a slit illuminator.
In most reduction stepper systems it is clear that magnification can be varied, by simply moving the reticle relative to the lens, and then refocusing the image on the wafer since most reduction steppers are not telecentric on the reticle side. However, this method results in the same amount of magnification variation in both the x and y directions. Since the scale of a wafer is likely to change slightly differently in the x direction than in the y direction, this is not an ideal solution.
Further, in known Wynne Dyson type optical projection systems it is only practical to view the alignment between the image and the pattern on the substrate by looking through the reticle when the reticle is not being illuminated with exposure radiation.
It would be desirable to have a system that permits the adjustment of the magnification of a lens system for small variations of magnification that result from various factors (e.g. variations in the temperature of the substrate, errors in the sphericity of optical surfaces within the system, etc.). The ability to modify the magnification factor along one axis of the final image, independently of the magnification factor for the orthogonal axis, would also be useful. Further, the ability to make magnification adjustments would be particularly useful in Wynne Dyson type optical systems where the magnification factor is 1:1 and not otherwise variable. Additionally, in a Wynne Dyson type optical system it would be useful to be able to view the image on a substrate at the image plane at any time, with or without the image of the reticle superimposed thereon. All of these improvements are provided by the present invention as will be seen from the following discussion and reference to the figures.